Low shrink telecommunications cable and methods for manufacturing the same

ABSTRACT

The present disclosure relates to a telecommunications cable having a layer constructed to resist post-extrusion shrinkage. The layer includes a plurality of discrete shrinkage-reduction members embedded within a base material. The shrinkage-reduction members can be made of a liquid crystal polymer. The disclosure also relates to a method for manufacturing telecommunications cables having layers adapted to resist post-extrusion shrinkage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/039,122, filed Jan. 18, 2005, which application is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to low shrinktelecommunications cable and to methods for manufacturing low shrinktelecommunications cable.

BACKGROUND

A fiber optic cable typically includes: (1) a fiber or fibers; (2) abuffer or buffers that surrounds the fiber or fibers; (3) a strengthlayer that surrounds the buffer or buffers; and (4) an outer jacket.Optical fibers function to carry optical signals. A typical opticalfiber includes an inner core surrounded by a cladding that is covered bya coating. Buffers typically function to surround and protect coatedoptical fibers. Strength layers add mechanical strength to fiber opticcables to protect the internal optical fibers against stresses appliedto the cables during installation and thereafter. Example strengthlayers include aramid yarn, steel and epoxy reinforced glass roving.Outer jackets provide protection against damage caused by crushing,abrasions, and other physical damage. Outer jackets also provideprotection against chemical damage (e.g., ozone, alkali, acids).

It is well known that micro-bending of an optical fiber within a cablewill negatively affect optical performance. Shrinkage of the outerjacket of a fiber optic cable can cause axial stress to be applied tothe optical fiber, which causes micro-bending of the optical fiber. Onecause of jacket shrinkage is thermal contraction caused by decreases intemperature. Another source of shrinkage is post-extrusion shrinkage.

Shrinkage caused by thermal contraction is typically only temporary. Theamount of thermal expansion/contraction is dependent upon thecoefficients of thermal expansion of the materials involved. In atypical fiber optic cable, the jacket has a higher coefficient ofthermal expansion than the fiber. Thus, when the temperature drops dueto normal environmental temperature cycling, the jacket may shrink morethan the fiber causing stresses to be applied to the fiber. Thesestresses are typically only temporary since the jacket will expand backto its original size when the temperature returns to normal.

Post-extrusion shrinkage is a by-product of the extrusion process usedto manufacture fiber optic cables. Generally, to make a fiber opticcable, an optical fiber is passed through an extrusion die and moltenplastic material is extruded about the exterior of the fiber. As themolten plastic exits the extrusion die, the plastic is elongated in thedirection of flow and then passed through a cooling bath where theelongated shape of the plastic is set. However, after the shape has beenset, the plastic material continues to have “memory” of thepre-elongated shape. Thus, if the cable is later heated, the plasticmaterial will gravitate towards its pre-elongated shape thereby causingpost-extrusion axial shrinkage of the cable jacket. As indicated above,cable jacket shrinkage can cause micro-bending of the optical fiberthereby degrading signal quality. Unlike shrinkage caused by thermalcontraction, post-extrusion shrinkage of the type described above ispermanent.

Post-extrusion shrinkage is a significant problem in the area of opticalfiber connectorization. When a connector is mounted to the end of afiber optic cable, a heat cure epoxy is often used to secure theconnector to the jacket and strength layer. When the epoxy is heatedduring the cure cycle, the cable jacket is also heated thereby causingpermanent post-extrusion shrinkage. Post-extrusion shrinkage can also becaused after installation by environmental temperature variations.

SUMMARY

One aspect of the present disclosure relates to a telecommunicationscable having a layer adapted to resist post-extrusion shrinkage. In oneembodiment, the layer is an outer jacket of the cable.

Another aspect of the present disclosure relates to a method for makinga telecommunications cable having a layer adapted to resistpost-extrusion shrinkage.

A variety of other aspects are set forth in the description thatfollows. The aspects relate to individual features as well as tocombinations of features. It is to be understood that both the foregoinggeneral description and the following detailed descriptions areexemplary and explanatory only and are not restrictive of the inventionas claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an example fiber optic cable inaccordance with the principles of the present disclosure;

FIG. 2 illustrates a layer of a telecommunications cable having aconstruction in accordance with the principles of the presentdisclosure;

FIG. 3 shows a second example of a fiber optic cable in accordance withthe principles of the present disclosure;

FIG. 4 shows a third example of a fiber optic cable in accordance withthe principles of the present disclosure;

FIG. 5 shows a fourth example of a fiber optic cable in accordance withthe principles of the present disclosure;

FIG. 6 illustrates a system for manufacturing telecommunications cablesin accordance with the principles of the present disclosure;

FIG. 6A is a cross-sectional view taken along section line 6A-6A of FIG.6;

FIG. 6B is a cross-sectional view taken along section line 6B-6B of FIG.6; and

FIG. 7 shows an example crosshead that can be used with the system ofFIG. 6.

DETAILED DESCRIPTION

The present disclosure relates generally to telecommunications cablelayers (e.g., jackets, buffers, sheaths, etc.) constructed to resistpost-extrusion shrinkage while maintaining flexibility. In oneembodiment, the layer comprises a jacket surrounding one or more tightbuffered optical fibers. In another embodiment, the layer can comprisean outer jacket that surrounds a copper cable. In still anotherembodiment, the layer can comprise a buffer tube for a loose-bufferedcable. In still another embodiment, the layer comprises a tight bufferlayer surrounding one or more optical fibers. While example applicationshave been listed above, it will be appreciated that layers in accordancewith the principles of the present disclosure can be used for any layerof a telecommunications cable where reduced shrinkage and relativelyhigh levels of flexibility are desirable.

FIG. 1 illustrates a fiber optic cable 20 that may incorporate one ormore shrinkage resistant layers in accordance with the principles of thepresent disclosure. The fiber optic cable 20 includes an optical fiber22, a buffer 24, a strength layer 26 and an outer jacket 28. The outerjacket 28 and/or the buffer 24 may have a construction adapted to resistpost-extrusion shrinkage.

It will be appreciated that the optical fiber 22 can have any number ofconventional configurations. For example, the optical fiber 22 mayinclude a silica-based core surrounded by a silica-based cladding havinga lower index of refraction than the core. One or more protectivepolymeric coatings may surround the cladding. The optical fiber 22 maybe a single-mode fiber or a multi-mode fiber. Example optical fibers arecommercially available from Corning Inc. of Corning, N.Y.

The buffer 24 is depicted as a tight buffer layer that surrounds thefiber 22. It will be appreciated that the buffer 24 can have any numberof conventionally known constructions. For example, the buffer 24 can bemade of a polymeric material such as polyvinyl chloride (PVC). Otherpolymeric materials (e.g., polyethylenes, polyurethanes, polypropylenes,polyvinylidene fluorides, ethylene vinyl acetate, nylon, polyester, orother materials) may also be used. In certain embodiments, the bufferlayer may have a construction adapted to resist post-extrusionshrinkage. For example, similar to the outer jacket 28 described below,the buffer can include shrinkage reduction members embedded therein toresist axial shrinkage.

The strength layer 26 is adapted to inhibit axial tensile loading frombeing applied to the optical fiber 22. The strength layer 26 preferablyextends the entire length of the fiber optic cable. In certainembodiments, the strength layer can include yarns, fibers, threads,tapes, films, epoxies, filaments or other structures. In a preferredembodiment, the strength layer 26 includes aramid yarns (e.g., Kevlar®yarns) that extend lengthwise along the entire length of the cable. Asdepicted in FIG. 1, the strength layer 26 is provided generally at theinterface between the buffer 24 and the jacket 28.

Referring to FIG. 2, the jacket 28 has a structure adapted to resistpost-extrusion shrinkage. For example, the jacket 28 includes a basematerial 30 and a plurality of discrete shrinkage-reduction members 32(e.g., rods, tendrils, extensions, fibers, etc.) embedded within thebase material 30. The shrinkage-reduction members 32 are preferablyconstructed of a material that has better post-extrusion shrinkcharacteristics than the base material 30. As described in thebackground, when the base material is stretched, the base materialretains a memory of the pre-stretched shape and will gravitate towardsthe pre-stretched shape when reheated. The shrinkage-reduction memberspreferably demonstrate less shrinkage than the base material whenreheated. Because the shrinkage-reduction members are embedded in thebase material, the shrinkage-reduction members provide reinforcementthat resists shrinkage of the base material. In a preferred embodiment,the shrinkage reduction material has a melting temperature that isgreater than the melting temperature of the base material.

Referring still to FIG. 2, the shrinkage-reduction members 32 arepreferably elongated and have lengths that are aligned generallyparallel to a longitudinal axis L-A of the cable 20. Each of theshrinkage reduction members 32 preferably does not extend the entirelength of the cable 20. Instead, each of the members 32 preferablycoincides with or extends along only a relatively short segment of thetotal length of the cable. For example, in one embodiment, at least someof the members 32 have lengths in the range of 0.2 mm-100 mm. In anotherembodiment, at least some of the members 32 have lengths in the range of5-60 mm. In still another embodiment, at least some of the members havelengths in the range of about 10-40 mm. In certain embodiments, amajority of the shrinkage reduction members provided within the basematerial can be within the size ranges provided above, or within othersize ranges. Additionally, most of the members 32 are preferablydiscrete or separate from one another. For example, many of the members32 are preferably separated or isolated from one another by portions ofthe base material 30.

To further promote flexibility, the concentration of theshrink-reduction members is relatively small as compared to the basematerial. For example, in one embodiment, the shrink-reduction materialconstitutes less than 2% of the total weight of the jacket 28. Inanother embodiment, the shrink-reduction material constitutes less than1.5% of the total weight of the jacket 28. In still another embodiment,the shrink-reduction material constitutes less than or equal to 1.25% ofthe total weight of the jacket 28. In a further embodiment, theshrink-reduction material constitutes less than or equal to 1.0% of thetotal weight of the jacket 28. While preferred embodiments use less than2% of the shrink-reduction material by weight, other embodiments withinthe scope of the present invention can use more than 2% by weight of theshrink-reduction material.

In one embodiment, the base material is a polymer such as a flexiblechain polymer (i.e., one in which successive units of the polymer chainare free to rotate with respect to one another, so that the polymerchain can assume a random shape). Example base materials includeconventional thermoplastic polymers such as polyethylene, polypropylene,ethylene-propylene, copolymers, polystyrene, and styrene copolymers,polyvinyl chloride, polyamide (nylon), polyesters such as polyethyleneterephthalate, polyetheretherketone, polyphenylene sulfide,polyetherimide, polybutylene terephthalate, low smoke zero halogenspolyolefins and polycarbonate, as well as other thermoplastic materials.Additives may also be added to the base material. Example additivesinclude pigments, fillers, coupling agents, flame retardants,lubricants, plasticizers, ultraviolet stabilizers or other additives.The base material can also include combinations of the above materialsas well as combinations of other materials.

In one embodiment, the shrinkage-reduction members are made from amaterial that can be softened and reshaped in the extrusion process. Ina preferred embodiment, the shrinkage-reduction members include liquidcrystal polymers. Example liquid crystal polymers are described in U.S.Pat. Nos. 3,991,014; 4,067,852; 4,083,829; 4,130,545; 4,161,470;4,318,842; and 4,468,364, which are hereby incorporated by reference intheir entireties. Liquid crystal polymers are polymers that areanisotropic and highly oriented, even in a softened or liquid phase.

In one embodiment, the jacket 28 shrinks less than 3% in length whenexposed to 110 degrees Celsius for 2 hours in accordance with standardTelcordia test procedures set forth at GR 409 (Generic Reference 409developed by Telcordia). In another embodiment, the jacket 28 shrinksless than 2% in length, or less than 1% in length, when subjected to thesame test. The amount of shrinkage is directly dependent on the amountof liquid crystal polymer used. Typically, when 2% liquid crystalpolymer by weight is used, the jacket length shrinks less than 1% onaverage. The above data is based on tests performed on jackets alonewith the fibers and strength members removed prior to shrink-testing.

FIG. 3 shows a two-fiber zipcord cable 220 having two optical fibers222, two buffers 224, two aramid strength layers 226, and an outerjacket 228. The outer jacket 228 preferably has a shrink-resistantconstruction of the type described with respect to the jacket 28 of FIG.2.

FIG. 4 shows a distribution cable 320 having a central strength member321, a plurality of optical fibers 322, buffers 324 surrounding each ofthe optical fibers, a tensile strength member 326, and an outer jacket328. The outer jacket 328 preferably has a shrink-resistant constructionof the type described with respect to the jacket 28 of FIG. 2.

FIG. 5 shows a loose buffered cable 420 having a central strength member421, bundles of unbuffered optical fibers 422 contained within buffertubes 424, a tensile strength layer 426, an inner sheath 427, anoptional armor layer 429, and an outer optional sheath 431. Tensilestrength layers 423 are also shown between the fibers 422 and the buffertubes 424. The fibers 422 are arranged around central strength members433 positioned within the buffer tubes 424. The buffer tubes 424 have ashrink-resistant construction of the type described with respect to thejacket 28 of FIG. 2

FIG. 6 illustrates a system 100 for making the fiber optic cable 20 ofFIG. 1. The system 100 includes a crosshead 102 that receivesthermoplastic material from an extruder 104. A hopper 106 is used tofeed materials into the extruder 104. A first conveyor 108 conveys thebase material to the hopper 106. A second conveyor 110 conveys theshrinkage-reduction material to the hopper 106. The extruder 104 isheated by a heating system 112 that may include one or more heatingelements for heating zones of the extruder as well as the crosshead todesired processing temperatures. Buffered optical fiber is fed into thecrosshead 102 from a feed roll 114 (see FIG. 6A). Strength members arefed into the crosshead from one or more feed rolls 116 (see FIG. 6A). Awater trough 118 is located downstream from the crosshead 102 forcooling the extruded product (see FIG. 6B) that exits the crosshead 102.The cooled final product is stored on a take-up roll 120 rotated by adrive mechanism 122. A controller 124 coordinates the operation of thevarious components of the system 100.

In use of the system 100, the base material and the shrinkage-reductionmaterial are delivered to the hopper 106 by the first and secondconveyors 108, 110, respectively. In certain embodiments, the basematerial and the shrinkage-reduction material can be delivered to thehopper 106 in pellet form, and the conveyors 108, 110 can includeconveyor belts or screw augers. The controller 124 preferably controlsthe proportions of the base material and the shrinkage-reductionmaterial delivered to the hopper 106. In one embodiment, theshrinkage-reduction material constitutes less than 2% by weight of thetotal material delivered to the hopper 106. In other embodiments, theshrinkage reduction material constitutes less than 1.5% of the totalweight of material delivered to the hopper 106. In still otherembodiments, the shrinkage reduction material constitutes less than orequal to 1% of the total weight of material delivered to the hopper 106.

From the hopper 106, the material moves by gravity into the extruder104. In the extruder 104, the material is mixed, masticated, and heated.In one embodiment, the material is heated to a temperature greater thanthe melting temperature of the base material, but less than the meltingtemperature of the shrinkage reduction material. The temperature ispreferably sufficiently high to soften the shrinkage-reduction materialsuch that the shrinkage-reduction material is workable and extrudable.The extruder 104 is heated by the heating system 112. The extruder 104also functions to convey the material to the crosshead 102, and toprovide pressure for forcing the material through the crosshead 102.

Referring to FIG. 7, the extruder 104 is depicted as including anextruder barrel 140 and an auger/style extruder screw 142 positionedwithin the barrel 140. An extruder screen 144 can be provided at theexit end of the extruder 104. The screen 144 prevents pieces too largefor extrusion from passing from the extruder into the crosshead 102.

Referring still to FIG. 7, the crosshead 102 includes a jacket materialinput location 200 that receives thermoplastic material from theextruder 104. The crosshead 102 also includes a tip 202 and a die 204.The tip 202 defines an inner passageway 206 through which the bufferedoptical fiber and the strength members are fed. The die 204 defines anannular extrusion passage 208 that surrounds the exterior of the tip202. The crosshead 102 defines an annular passageway for feeding thethermoplastic jacket material to the annular extrusion passage 208.Within the crosshead, the flow direction of the thermoplastic materialturns 90 degrees relative to the flow direction of the extruder 104 toalign with the buffered fiber.

Within the crosshead 102, the material provided by the extruder 104 ispreferably maintained at a temperature greater than the melt temperatureof the base material, but less than the melt temperature of theshrinkage reduction material. As the thermoplastic material is extrudedthrough the annular extrusion passage 208, the base material and theshrinkage-reduction material are stretched. This stretching causesreshaping of the shrinkage-reduction material into elongatedshrinkage-reduction members having lengths aligned generally along thelongitudinal axis of the fiber optic cable. The extruded fiber opticcable is then cooled and shape set at the water trough 118. Theextrusion process can be a pressure or semi-pressure extrusion processwhere product leaves the crosshead at the desired shape, or an annularextrusion process where the product is drawn down after extrusion. Aftercooling, the product is collected on the take-up roller 120.

EXAMPLES

This invention will now be further described in detail with reference toa specific example. It will be understood that this example provides oneembodiment of the invention and is not intended to limit the scope ofthe invention.

One experimental example used Dow 1638 low smoke zero halogen materialas a base material mixed with a liquid crystal polymer such as TiconaVectra A950. The base material has a melt temperature of 362° F., andthe liquid crystal polymer has a melt temperature of 536° F. and asoftening temperature of 293° F. The materials were mixed at a ratio of99% base material and 1% of the liquid crystal polymer. The materialswere masticated within a screw extruder and heated to a temperature of452° F. The materials were then forced through a crosshead including atip having an outside diameter of 0.062″, and a die defining anextrusion opening having an inside diameter of 0.130″. The crosshead washeated to a temperature of 500° F. The run speed was 12 meters perminute. The extruded jacket had an exterior diameter of 0.0787″ (2.0 mm)and an interior diameter of 0.061″ (1.54 mm). The jacket was extrudedwithout an inner optical core. After cooling, the jacket was cut into150 mm segments and heated to 110 degrees Celsius for 2 hours to testfor shrinkage. The testing showed that the jacket segments shrunk lessthan 2% in length on average based on GR 409 shrink testing.

Another experimental example used Dow 1638 low smoke zero halogenmaterial as a base material mixed with a liquid crystal polymer such asTicona Vectra A950. The base material has a melt temperature of 362° F.,and the liquid crystal polymer has a melt temperature of 536° F. and asoftening temperature of 293° F. The materials were mixed at a ratio of98% base material and 2% of the liquid crystal polymer. The materialswere masticated within a screw extruder and heated to a temperature of475° F. The materials were then forced through a crosshead including atip having an outside diameter of 0.064″, and a die defining anextrusion opening having an inside diameter of 0.127″. The crosshead washeated to a temperature of 500° F. The run speed was a high speed run of143 meters per minute. The extruded jacket had an exterior diameter of0.0787″ (2.0 mm) and an interior diameter of 0.0508″ (1.29 mm). Thejacket was extruded about an internal optical core. After cooling, thejacket was cut into 150 mm segments and heated to 110 degrees Celsiusfor 2 hours to test for shrinkage. The testing showed that the jacketsegments shrunk less than 1% in length on average based on GR 409 shrinktesting. The core was removed for the shrink testing.

Since many embodiments of the invention can be made without departingfrom the spirit and scope of the invention, the invention resides in theclaims hereinafter appended and the broad inventive aspects underlyingthe specific embodiments disclosed herein.

1. A method for making a layer that surrounds an optical fiber, themethod comprising: providing a base material suitable for use in a layerthat surrounds an optical fiber; adding liquid crystal polymer materialto the base material to form a mixture that is adapted to reducepost-extrusion shrinkage, wherein the liquid crystal polymer material isadded in an amount that is less than 2% of the total weight of themixture; heating the mixture to a first temperature; extruding themixture through an annular passage; and cooling the mixture to form thelayer having a desired annular shape, wherein the layer shrinks lessthan 3% in length when exposed to 110 degrees Celsius for two hours inaccordance with standard Telcordia test procedures set forth at GenericReference
 409. 2. A method for making a layer that surrounds an opticalfiber as claimed in claim 1, wherein the liquid crystal polymer has amelt temperature that is greater than the melt temperature of the basematerial and a softening temperature that is less than the meltingtemperature of the base material.
 3. A method for making a layer thatsurrounds an optical fiber as claimed in claim 1, wherein the layer is atight buffer layer.
 4. A method for making a layer that surrounds anoptical fiber as claimed in claim 1, wherein the layer is a buffer tube.5. A method for making a layer that surrounds an optical fiber asclaimed in claim 1, wherein the layer is an outer jacket.
 6. A methodfor making a layer that surrounds an optical fiber as claimed in claim1, wherein the liquid crystal polymer material forms a plurality ofdiscrete, elongated shrinkage reduction members.
 7. A method for makinga layer that surrounds an optical fiber as claimed in claim 6, whereinat least some of the shrinkage reduction members have lengths in therange of 0.2 mm to 100 mm.
 8. A method for making a layer that surroundsan optical fiber as claimed in claim 6, wherein at least some of theshrinkage reduction members have lengths in the range of 5 mm to 60 mm.9. A method for making a layer that surrounds an optical fiber asclaimed in claim 6, wherein at least some of the shrinkage reductionmembers have lengths in the range of 10 mm to 40 mm.
 10. A method formaking a layer that surrounds an optical fiber as claimed in claim 1,wherein the layer shrinks less than 2% in length when exposed to 110degrees Celsius for two hours in accordance with standard Telcordia testprocedures set forth at Generic Reference
 409. 11. A method for making alayer that surrounds an optical fiber as claimed in claim 1, wherein thelayer shrinks less than 1% in length when exposed to 110 degrees Celsiusfor two hours in accordance with standard Telcordia test procedures setforth at Generic Reference 409.